There has been known a hollow poppet valve having a valve head and a stem portion integral therewith and formed with an internal cavity that extends from within the valve head into the stem portion, the cavity being charged, together with an inner gas, with a coolant that has a higher heat conductivity than the valve material. An example of such coolant is metallic sodium which has a melting point of about 98° C.
Since this valve has an internal cavity extending from within the valve head into the stem portion, and hence larger than a conventional internal cavity formed only in a stem portion, the valve can contain a larger volume of coolant in the internal cavity than the conventional valve and provide an enhanced heat transferability (hereinafter referred to as heat reduction capability). Moreover, when the valve has a diametrically larger internal cavity in the valve head than a diametrically small internal cavity in the stem portion, it can be charged with a still larger volume of coolant in the valve to increase its heat reduction capability.
It is known that if a combustion chamber of an engine is heated to an excessively high temperature during an operation, knocking may take place, which lowers the fuel efficiency, and hence the performance, of the engine. In order to lower the temperature of the combustion chamber, there has been proposed different types of hollow poppet valves each having an internal cavity filled with a coolant together with an inert gas so as to positively remove heat from the combustion chamber by enhanced heat reduction capability of the poppet valve.
Patents Documents 1 and 2 listed below disclose methods of manufacturing hollow poppet valves in which metallic sodium is injected from a nozzle into an internal cavity of the valve placed below the nozzle. Particularly, Patent Document 2 discloses a step of injecting pressurized clayish metallic sodium extruded from a nozzle of an extruder into a cavity of a valve below the nozzle while cooling the metallic sodium.